The use of murine models to investigate human cutaneous oncology, immunology and keratinocyte biology is advantageous over the use of human skin for obvious reasons. However, substantial differences exist between human skin and murine skin. In human skin, Stem Cell Factor is produced by epidermal keratinocytes after birth, unlike in normal murine skin. The result of this, among other things, is that melanocytes are present in the interadnexal epidermis in human skin. In contrast, melanocytes in adult murine skin are generally confined to hair follicles, with the exception of rare epidermal melanocytes found in the ears, footpads, and tail (1). A few dermal melanocytes may also be found in mice, mostly in the ears. These differences have compromised the use of the mice as a model system for investigation of human cutaneous biology.
It has been discovered that melanocyte migration and development, as well as the survival of melanocytes and mast cells, are dependent on expression of the kit protein, a receptor tyrosine kinase encoded by the c-kit proto-oncogene (2–6). The ligand for kit, known as stem cell factor (SCF) (also called mast cell growth factor, steel factor, and kit ligand) may be produced locally in human skin by epidermal keratinocytes, fibroblasts, and endothelial cells (7–8). However, definitive studies of SCF production in murine skin have not been reported. Transgenic studies using the SCF gene promoter region and beta-galactosidase as a reporter gene suggest that, unlike in human skin, postnatal murine cutaneous SCF expression is limited to the dermis and hair follicles, and not found in epidermal keratinocytes (9). The difference in SCF expression between human and murine epidermis could explain the difference in melanocyte distribution and other biological phenomena in these two species.
SCF may be produced in two isoforms by alternate splicing of exon 6. One isoform lacks exon 6 encoded sequences and exists predominantly as a membrane-bound molecule. The other isoform contains exon 6 encoded sequences which include a protease sensitive site (10–19). Cleavage at the protease sensitive site causes the release of a soluble, bioactive form of SCF. The membrane-bound and soluble forms of SCF have differential effects on melanocyte precursor dispersal and survival (20) and exogenous soluble SCF may produce cutaneous mast cell hyperplasia and cutaneous hyperpigmentation (21–23). In addition, local high concentrations of soluble SCF have been found in lesions of human cutaneous mastocytosis, a disease characterized by dermal accumulations of mast cells and increased epidermal melanin (7, 8, 24) and in spongiotic dermatitis, a common inflammatory condition of human skin (our unpublished data).